ExtremeTech explains: What is LTE?

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It’s been several years since the first LTE networks came online. Now, nearly all cellular-enabled devices sold today support LTE for 4G service — sometimes even without 2G or 3G technologies supported. The first LTE-compatible phones only had a few hours of practical battery life, but today’s devices can last an entire day or two on a single charge. That’s still not enough, of course, but we’re getting there.

So, what is LTE? To most, it is a faster network technology. To network operators around the world, it is a way to simplify their infrastructures to reduce costs while improving the quality of their offerings to subscribers. Advertisements by network operators declare it as the “most advanced” network technology. In the end, it is Long Term Evolution of the Universal Mobile Telecommunications System (UMTS).

But that doesn’t tell us what LTE actually is. LTE is what the 3GPP (3rd Generation Partnership Project, the group responsible for standardizing and improving UMTS) designates as their next step. UMTS is the group of standards that define 3G for GSM networks across the world, including AT&T and T-Mobile’s 3G networks. The cdmaOne/CDMA2000 family of standards are not maintained by 3GPP, but by a different organization spearheaded by Qualcomm. For subscribers to operators with networks utilizing CDMA2000 technology, LTE is the replacement of mediocre CDMA2000 networks with a superior cellular telecommunications system offering flexibility and power to the network operator and the subscriber.

LTE is a very good, easily deployable network technology, offering high speeds and low latencies over long distances. For example, two of the four operators’ LTE networks in New York City were rated well for achieving this goal. Verizon’s LTE service was rated with an average download speed of 31.1Mbps and an average upload speed of 17.1Mbps. T-Mobile’s LTE service was rated with an average download speed of 20.5Mbps and an average upload speed of 13.5Mbps.

Of course, that doesn’t mean all networks are created equal. Some aren’t quite able to achieve these goals. For example, Sprint’s LTE service was rated with an average download speed of 4.0Mbps and an average upload speed of 2.5Mbps. AT&T’s LTE service was much better than Sprint’s, but still bad with an average download speed of 7.6Mbps and an average upload speed of 2.4Mbps.

In this article, we will discuss what configurations LTE can be deployed in, why LTE is easily deployable, how LTE works as a radio technology, what types of LTE exist, how LTE affects battery life, what network operators want LTE to do, and the future of 4G as a whole. The most technical parts of the article are LTE can be deployed in, why LTE is easily deployable, how LTE works as a radio technology, and what types of LTE exist. For those who don’t want that information, you can skip to how LTE affects battery life and still get the gist of what we’re saying. But to get the complete picture, reading the whole article is advised.

How LTE is configured for deployment

LTE supports deployment on different frequency bandwidths. The current specification outlines the following bandwidth blocks: 1.4MHz, 3MHz, 5MHz, 10MHz, 15MHz, and 20MHz. Frequency bandwidth blocks are essentially the amount of space a network operator dedicates to a network. Depending on the type of LTE being deployed, these bandwidths have slightly different meaning in terms of capacity. That will be covered later, though. An operator may choose to deploy LTE in a smaller bandwidth and grow it to a larger one as it transitions subscribers off of its legacy networks (GSM, CDMA, etc.).

MetroPCS was an example of a network operator that has done this. Before it was acquired by T-Mobile, a majority of its spectrum is still dedicated to CDMA, with 1.4MHz or 3MHz dedicated for LTE depending on the market. There were a few markets with 5MHz deployed, but these were the exception, not the rule. Leap Wireless (who did business as Cricket Communications) had also done the same thing prior to being acquired by AT&T, except it used 3MHz or 5MHz instead of 1.4MHz or 3MHz. Neither of these operators could afford to cut CDMA capacity by a significant degree just yet, so LTE operated on tiny bandwidths. Additionally, neither operator had enough backhaul (the core network infrastructure and connections to the internet) dedicated to LTE to make larger bandwidths worth it either. Of course, these issues went away when they were acquired. MetroPCS and Cricket transitioned service to the T-Mobile and AT&T networks, respectively. Their networks are being wound down and their spectrum is redeployed to support their new parent companies’ GSM/UMTS/LTE networks.

On the other hand, Verizon Wireless has been using 10MHz wide channels for LTE all across the board for 750MHz, since it has the national allocation of spectrum available for it. In addition to that, the AWS spectrum it acquired from the cable companies and other transactions have allowed it to roll out a second LTE pipeline with 15MHz or 20MHz channels in most places. Like Verizon, T-Mobile is also rolling out wide channels for LTE on it’s AWS spectrum. Combined with excellent backhaul, LTE service from those two companies promise to be best in class. On AT&T’s side, LTE channel sizes vary depending on the market. In most markets, AT&T has 10MHz channels on 700MHz, but there are many where it only has 5MHz. It has resorted to cutting down GSM capacity to reuse the spectrum to support its customers, as singular 5MHz or even 10MHz channels aren’t enough. Sprint has a similar problem, as its main network is a singular 5MHz channel nationally. It is using the spectrum it has from acquiring Clearwire to supplement it with 20MHz channels for additional capacity.

Less spectrum means that fewer customers can obtain the same high speeds that Verizon’s LTE customers get when connected to any particular cell. LTE can support up to 200 active data clients (smartphones, tablets, USB modems, mobile hotspots, etc.) at full speed for every 5MHz of spectrum allocated per cell. That means that if a particular tower has 20MHz of spectrum allocated to it, it can support up to 800 data clients at full speed. There are ways of supporting more data clients per 5MHz, but doing so requires sacrificing speed and capacity, as the 200-per-5MHz ratio is the optimal configuration. However, spectrum isn’t everything to LTE quality, as I will discuss later.

How LTE actually works

LTE uses two different types of air interfaces (radio links), one for downlink (from tower to device), and one for uplink (from device to tower). By using different types of interfaces for the downlink and uplink, LTE utilizes the optimal way to do wireless connections both ways, which makes a better-optimized network and better battery life on LTE devices.

For the downlink, LTE uses an OFDMA (orthogonal frequency division multiple access) air interface as opposed to the CDMA (code division multiple access) and TDMA (time division multiple access) air interfaces we’ve been using since 1990. What does this mean? OFDMA (unlike CDMA and TDMA) mandates that MIMO (multiple in, multiple out) is used. Having MIMO means that devices have multiple connections to a single cell, which increases the stability of the connection and reduces latency tremendously. It also increases the total throughput of a connection. We’re already seeing the real-world benefits of MIMO on WiFi N routers and network adapters. MIMO is what lets 802.11n WiFi reach speeds of up to 600Mbps, though most advertise up to 300-400Mbps. There is a significant disadvantage though. MIMO works better the further apart the individual carrier antennae are. On smaller phones, the noise caused by the antennae being so close to each other will cause LTE performance to drop. WiMAX also mandates the usage of MIMO since it uses OFDMA as well. HSPA+, which uses W-CDMA (a reworked, improved wideband version of CDMA) for its air interface, can optionally use MIMO, too.

For the uplink (from device to tower), LTE uses the DFTS-OFDMA (discrete Fourier transform spread orthogonal frequency division multiple access) scheme of generating a SC-FDMA (single carrier frequency division multiple access) signal. As opposed to regular OFDMA, SC-FDMA is better for uplink because it has a better peak-to-average power ratio over OFDMA for uplink. LTE-enabled devices, in order to conserve battery life, typically don’t have a strong and powerful signal going back to the tower, so a lot of the benefits of normal OFDMA would be lost with a weak signal. Despite the name, SC-FDMA is still a MIMO system. LTE uses a SC-FDMA 1×2 configuration, which means that for every one antenna on the transmitting device, there’s two antennae on the base station for receiving.

The major difference between the OFDMA signal for downlink and the SC-FDMA signal for uplink is that it uses a discrete Fourier transform function on the data to convert it into a form that can be used to transmit. Discrete Fourier transform functions are often used to convert digital data into analog waveforms for decoding audio and video, but it can be used for outputting the proper radio frequencies too. However, LTE-Advanced uses higher order MIMO configurations for downlink and uplink.

The LTE technology itself also comes in two flavors: an FDD (frequency division duplex) variant and a TDD (time division duplex) variant. The most common variant being used is the FDD variant. The FDD variant uses separate frequencies for downlink and uplink in the form of a band pair. That means for every band that a phone supports, it actually uses two frequency ranges. These are known as paired frequency bands. For example, Verizon’s 10MHz network is in FDD, so the bandwidth is allocated for uplink and downlink. This is commonly noted as a 2x10MHz or 10+10 MHz configuration. Some also call it 10x10MHz, but this is mathematically incorrect, but they mean 10+10MHz. Some will also call it a 20MHz network, but this can be ambiguous. The TDD variant uses one single range of frequencies in a frequency band, but that band is segmented to support transmit and receive signals in a single frequency range.

For example, an LTE TDD network deployed on 20MHz of spectrum uses the whole chunk as one large block for frequency allocation purposes. For network bandwidth purposes, a LTE TDD network’s spectrum can be further divided to optimize for the type of network traffic (half up and half down, mostly down and a bit up, mostly up and a bit down, and so on).

In the United States, Sprint is the only network operator deploying LTE in the TDD variant. Everyone else is deploying in the FDD variant. The TDD variant becomes more important in Asia, as China Mobile (the largest network operator in the world in terms of subscriber count) is using TDD frequencies for their LTE network. Sprint’s parent company, SoftBank, also uses LTE TDD in its home market of Japan. Fortunately, LTE devices can easily be made to support both variants on a device without too much trouble.

Enough about specs – what about battery life?

Now we lead to the part that most people care about: how it affects battery life. By itself, LTE devices should last roughly as long as their HSPA+ equivalents because of the optimized radios for both downlink and uplink operations. The reason why LTE devices right now eat batteries for breakfast is because the network operators are forcing many of these devices into active dual-mode operation.

For Verizon Wireless, this means that most of their LTE devices connect to both CDMA2000 and LTE simultaneously and stay connected to both. This means that you are eating twice the amount of battery for every minute you are connected than you would if you were connected only to CDMA2000 or LTE. Additionally, when you make calls on Verizon Wireless LTE phones, the CDMA2000 radio sucks down more power because you are talking. Sending and receiving text messages causes pulses of CDMA2000 activity, which cuts your battery life more. Arguably, constantly changing radio states could be worse for battery life than a switch into one mode for a period of time and switching back, so text messages may actually kill the batteries faster.

Then there is handover. Handover is the operation in which a device switches from one network to another or from one tower to another. Handover is the critical component that makes any cellular wireless network possible. Without handover, a user would have to manually select a new tower every time the user leaves the range of a tower. (WiFi is an example of a wireless network technology that doesn’t inherently support handover.) When the user travels outside the range of a WiFi network, the WiFi radio will just drop the connection. For cellular networks, this is even more critical because the range of a tower is not very predictable due to factors outside of anyone’s control (like the weather, etc.). LTE supports handover like all other cellular wireless networks, but it improves on it by doing it much faster when handing over to a supported type of network or cell.

However, Verizon and Sprint are doing handover from LTE to EV-DO and back by plugging in a connection to an enhanced version of the EV-DO data network core called eHRPD. This isn’t a great solution by any means.

The fragile link-up between EV-DO and LTE make handover occur a lot more than it is supposed to, which eats battery life even more. With AT&T and T-Mobile using an HSPA+ network alongside LTE instead of CDMA2000, handover operation is a lot smoother. As far as battery life goes, it should be slightly better than Verizon and Sprint LTE phones because LTE supports fast handover between UMTS and LTE. AT&T LTE phones are normally not forced into active dual-mode operation because HSPA+ lets you use data and talk at the same time. As a consequence, AT&T has no need to force the device into active dual-mode operation. However, battery life will still be pretty bad because LTE signals are still very weak in most AT&T LTE zones, and AT&T LTE devices default to connecting to LTE signals whenever possible.

C Spire Wireless, U.S. Cellular, and other CDMA/LTE operators all have the same problem as Verizon Wireless with LTE battery life because they do the same thing as Verizon Wireless and force active dual-mode operation with most of their devices. As a result, turning off LTE will significantly improve battery life because the phone switches back to single-mode operation. Or in the case of AT&T phones, passive dual-mode operation (for GSM/HSPA+ handover) since they are typically in passive tri-mode operation for GSM/HSPA+/LTE handover. Passive multi-mode operation means that the device isn’t constantly connected to multiple networks, but will establish a connection and hand over the connection if the signal on the current network is too weak or snaps. This is ideal for multi-mode operation, and Sprint switched to doing this last year in order to control costs for new devices that support the tri-band LTE network it brands as “Sprint Spark”. With the launch of Verizon VoLTE last year, Verizon has also increasingly started offering devices with passive multi-mode operation. This means that these new devices now have many of the same energy conservation benefits that have always been present in GSM/UMTS/LTE devices.

LTE — Mobile panacea?

The ultimate goal of the network operators deploying LTE is to replace everything else they have with it. That means that it needs to become possible to handle voice calls, text messages, network alerts, etc. over the data network. However, no one developed the LTE specification with voice and text messaging in mind. It was designed as a data network only. So how do they solve the problem? By developing a VoIP solution that fits their needs. Two main standards came into existence: VoLGA (Voice over LTE via Generic Access) and VoLTE-IMS (Voice over LTE via IMS). VoLGA was based on GAN (Generic Access Network), which is also known as UMA (Unlicensed Mobile Access). Deutsche Telekom was the only network operator that wanted to use this method, as the design for VoLGA was heavily derived from T-Mobile USA’s implementation of UMA for its WiFi Calling feature. No one else wanting to deploy LTE wanted to use it as a final or interim solution, as it would have meant keeping around the legacy GSM core network for this purpose.

Everyone else supported VoLTE-IMS (now referred to as VoLTE), which allowed them to fully discard their older networks and simplify their network design as they decommissioned legacy networks. However, IMS is much more expensive and difficult to deploy than VoLGA, at least for GSM network operators. But IMS also promised more flexibility. IMS could be used to make real-time video calling with all sorts of additional features possible. And so, Deutsche Telekom dropped VoLGA and joined everyone else in supporting VoLTE.

VoLTE uses an extended variant of SIP (Session Initiation Protocol) to handle voice calls and text messages. For voice calls, VoLTE uses the AMR (Adaptive Multi-Rate) codec, with the wideband version used if supported on the network and the device. The AMR codec has long since been used as the standard codec for GSM and UMTS voice calls. The wideband version supports higher quality speech encoding, which would allow for clearer voice calls. Text messages are supported using SIP MESSAGE requests. Video calling uses H.264 CBP (constrained baseline profile) with AMR-WB audio codec over RTP (Real-time Transport Protocol) with VBR (variable bit rate). With this, video calls over IMS are supposed to be very high quality, no matter what the quality of the data connection. With VBR, the call can adapt to the changing congestion levels of the data network to maintain a quality video call.

MetroPCS was one of the first carriers in the world to deploy VoLTE, the other being SK Telecom in South Korea. While SK Telecom launched VoLTE nationally, MetroPCS only launched VoLTE in Dallas, TX. Initial implementations of VoLTE clearly showed much lower battery life, but that is largely resolved now. After acquiring MetroPCS, T-Mobile reconfigured its two networks for broadly rolling out LTE service and shutting down the CDMA service it inherited from MetroPCS. At the end of the summer of 2014, T-Mobile launched VoLTE and made it available nationwide shortly after the launch. Since it is based on their WiFi calling solution, it is able to support seamless handover from WiFi to LTE and back for voice calls, which no one else has done.

At this point, the major gotcha with VoLTE now is that it requires carrier customized firmware. Setting up a device to use VoLTE requires a lot of configuration, more than what a SIM card can provide. Consequently, only operator branded devices will support VoLTE for now. Some unbranded devices may eventually be preloaded with select operator VoLTE configuration information, but that will remain the exception rather than the rule. Hopefully this particular problem will be addressed soon, because it puts a crimp in any plan to move devices from one operator to another.

The messy future of 4G

We’ve only scratched the surface of what LTE is all about, but this article includes pretty much everything that LTE subscribers would care about. Some of the other aspects of LTE include SON (self-organizing network) capabilities, which allows it to flexibly allocate capacity to parts of the cellular network as it is needed by redistributing connections to an optimal configuration at any given time. Handover to WiFi is another cool feature, too. However, most of the features like the former are pretty much only seen from a network operator’s side of things, and things like the latter may never actually be implemented.

Whether LTE becomes the success story of the mobile industry remains to be seen. Network operators around the world are only now deploying it, and already it is turning into a mess. The 3GPP has already approved nearly 45 frequency bands for LTE. Over 30 of them are for LTE FDD and the rest are for LTE TDD. Roaming is going to be very difficult on LTE. For North America alone, there are ten FDD bands and one TDD band for LTE. For Europe and Africa, there are four bands for FDD LTE and two bands for TDD LTE. For Asia and Oceania, there are the same four FDD bands for Europe, three more frequency bands for FDD, and two more TDD bands. The rest of the bands have yet to be used, but they are going to be used. Someone is going to have to figure out how to fit more bands on an LTE device without sacrificing portability. Fortunately, a number of bands are supersets/subsets of other bands, so some are easier to support than others.

Since LTE-Advanced Release 10 has been codified and equipment is available, a number of operators around the world have started using LTE-Advanced features. AT&T has deployed carrier aggregation in many of the same areas it has launched VoLTE, and Sprint intends to launch carrier aggregation in areas where the company has rolled out its tri-band “Spark” network. T-Mobile USA has been deploying and testing it since it started its rollout of LTE service so that it can maximize the benefits of LTE-Advanced equipment in as many areas as possible. While T-Mobile isn’t broadly using carrier aggregation yet, it certainly has the option to do so, in the future.

I don’t know what the future holds for LTE, but it will certainly be very interesting. This is the most exciting time in the mobile industry since the switchover from analog to digital back in the early 1990s. LTE represents a paradigm shift from hybrid voice and data networks to data-only networks. Going forward, wireless network technology is likely to become more widely used because it will become easier to obtain than wireline services (cable, DSL, etc.). It is doubtful that it would fully replace wireline data services though. Hopefully, the issues we face with LTE now will go away over time.

As LTE continues to improve, we’ll continue to see a steady migration of usage from older networks to LTE, especially operators using CDMA networks as a legacy technology. While we may not have ubiquitous connectivity for a long time due to the sheer number of bands and configurations, new technology geared to improve the situation is always coming.

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